The invention relates to a redundant or emergency steering mechanism for a craft, especially an aircraft to secure the functions of a steering system when an aircraft becomes unstable during a disturbed operation such as an emergency, in which a pressure medium, such as a hydraulic medium, is utilized as the signal carrier.
A hydraulic adjustment drive with an electric-hydraulic, open loop control utilizing a hydraulic piston as well as a mechanical control connected in parallel is disclosed in German Patent Publication No. DE-OS 1,800,866. This mechanical control is automatically engaged or becomes effective upon failure of the electric-hydraulic control. This arrangement is supposed to allow a smooth transition to the mechanical control and/or steering in all control positions or under all control conditions in case of failure of the electrical control. To accomplish this purpose, the control rod of the mechanical control system providing the control output signal is divided into two control rods interconnected by means of a lever linkage. The first rod acts as an actuator of a control slide valve for an adjustment piston and the second rod carries a trimming piston which is freely movable during normal operation. Both rods are coordinated by means of a locking or arresting coupling which alternately blocks one or the other control rod.
A purely manual "Back-up Steering Mechanism", as such redundant steering mechanisms are referred to in the art, which is supposed to secure the functions of an unstable aircraft during an emergency or disturbed operation has been found to be insufficient. An unstable aircraft may also be referred to as a "critical" automatic control system. For example, an unstable aircraft without additional stabilizing mechanisms cannot be controlled and/or steered manually.
It has also been suggested in the prior art to make the control paths redundant. However, this solution has proved to be useless, for example, upon failure of the electronic circuit means or upon failure of the electric power supply.